A light sensitive planographic printing plate precursor (hereinafter referred to also as a printing plate precursor) is known which comprises a support subjected to hydrophilization treatment and provided thereon, a photopolymerizable layer and a protective layer. Recently, in order to obtain a printing plate with high resolution image in a short time or to carry out a direct plate making system, a method has been applied in which a planographic printing plate precursor is digitally exposed based on image information employing laser rays, and developed with a developer to prepare a printing plate.
As one embodiment, there is a system preparing a planographic printing plate in which a light sensitive planographic printing plate precursor is scanning-exposed employing a light source modulated with an image signal transmitted by communication lines or output signals from an electronic plate making system or an image processing system.
Generally, a photopolymerizable layer contains an acryl monomer, an alkali soluble resin, a photo-initiator, and optionally a sensitizing dye to absorb a range of wavelengths of emitted light (particularly laser light).
As a light source for exposing a photopolymerization type light sensitive planographic printing plate precursor is used a visible light source having a longer wavelength such as an Ar laser (488 nm) or an FD-YAG laser (532 nm). In recent years, semiconductor lasers employing, for example, InGaN type material, which can continuously emit light with a wavelength of from 380 to 430 nm, are about to be put into practical use. A scanning exposure system employing light with such a short wavelength as a light source has advantages in providing sufficient output power and an economical system, since the semiconductor lasers can be structurally manufactured at low cost. Further, a light sensitive planographic printing plate precursor to be applied to the above light source has a spectral sensitivity shorter than that of a planographic printing plate precursor to be applied to a system employing a conventional FD-YAG laser or Ar laser, and therefore provides an excellent safe light property, whereby operation under room light is easy.
Herein, sensitivity of the light sensitive planographic printing plate precursor to short wavelength light is problem. Many proposals have been made regarding photopolymerizable light sensitive materials sensitive to a visible light with a wavelength exceeding 450 nm. For example, there are a combination of specific sensitive dyes disclosed I U.S. Pat. No. 2,850,445 and amines (JP 44-20189), a combination of hexaarylbiimidazoles and dyes (JP 45-37377), a combination of hexaarylbiimidazoles and p-dialkylaminobenzylideneketones (JP 45-2528 and Japanese Patent O.P.I. Publication No. 54-155292), a combination of cyclic-cis-α-dicarbonyl compounds and dyes (Japanese Patent O.P.I. Publication No. 48-84183), a combination of cyclictriazine compounds and merocyanine dyes (Japanese Patent O.P.I. Publication No. 54-151024), a combination of keto-cumarine and active agents (Japanese Patent O.P.I. Publication Nos. 52-112681 and 58-15503), a combination of bisimidazole, styrene derivatives and thiols (Japanese Patent O.P.I. Publication No. 59-140203), a combination of organic peroxides and dyes (Japanese Patent O.P.I. Publication Nos. 59-140203 and 59-189340), a combination of dyes with a rhodanine nucleus and radical generating agents (Japanese Patent O.P.I. Publication No. 2-244050), and a combination of aliphatic amine salts and pyrromethene borate dyes (Japanese Patent O.P.I. Publication No. 2000-250206).
It is disclosed in Japanese Patent O.P.I. Publication Nos. 59-152396, 61-151197, 63-10602, 63-41484, and 3-12403 that titanocenes are effective as a photopolymerization initiator. A combination of titanocenes and 3-ketocumarine dyes is disclosed in Japanese Patent O.P.I. Publication No. 63-221110, a combination of titanocenes, xanthene dyes and monomers having an amino group or a urethane group in Japanese Patent O.P.I. Publication Nos. 4-221958 and 4-219756, and a combination of titanocenes, and specific merocyanine dyes in Japanese Patent O.P.I. Publication Nos. 6-295061 and 9-328505.
However, these methods are not satisfactory in sensitivity.
A photopolymerization type light sensitive planographic printing plate precursor is imagewise exposed, optionally heated, washed with water (in order to remove a protective layer), developed with a developer to remove unexposed portions, washed with water, and post-processed with a finisher or a gumming solution to make non-image portions hydrophilic. Thus, a printing plate is obtained. When the protective layer at image portions is not sufficiently removed or finisher or gum components are firmly fixed to the image portions, ink receptivity of the image portions is insufficient, resulting in loss of printed paper sheets. As the countermeasure, a method is proposed in Japanese Patent O.P.I. Publication No. 10-315598, which controls a residual solvent content of the light sensitive layer before the protective layer is provided. However, this method is not satisfactory.
An aqueous alkaline solution having a pH of not less than 12.5 is generally used in order to remove the light sensitive layer at non-image portions, i.e., develop the light sensitive layer. In recent years, development employing an alkali developer having a lower pH has been required in view of workability, safety, or environmental concerns.
However, development of the conventional photopolymerizable light sensitive planographic printing plate precursor with a lower pH alkali developer has a problem in that developability is lowered, and sludge is likely to occur in the developer. Improvement of developability employing a low pH alkali developer is likely to result in lowering of ink receptivity and printing durability.
Further, when the conventional photopolymerizable light sensitive planographic printing plate precursor is exposed to laser, and developed with a developer while replenished with a developer replenisher for a long time without being exchanged to fresh developer, sludge occurs in the developer, and the resulting planographic printing plate has problems in that spotted stain or slight contamination occurs at non-image portions, and ink contamination at dot image portions during printing. A developer, after a large number of light sensitive planographic printing plate precursor was processed, lowers dot image reproduction, particularly reproduction of small dots, dots at shadows, and thin lines, wherein prints with stable quality cannot be obtained. Particularly when printing is carried out employing ink containing no petroleum volatile solvent for environmental reason, such lowering is likely to occur.